With the progress in pervasion of information communication equipment such as wireless terminals, frequencies available for communications have been made broadband dramatically from several millions of hertz for cellular phones or the like to several gigahertz for wireless LAN or the like. Under current conditions, terminals supporting various communication systems are used independently. It is, however, expected to realize a wireless terminal supporting various communication systems by itself in the future.
In addition, with the progress in miniaturization of wireless terminals, it has been desired to miniaturize passive parts such as signal selection devices built in housings of the terminals. Particularly signal selection devices using electric resonance in LC or the like often used in wireless communications are difficult to miniaturize because the size of resonators depends on the electric length. Due to this problem, in recent years, novel principles of signal selection have been searched.
Of them, RF-MEMS signal selection devices which can be manufactured by MEMS (Micro Electro Mechanical Systems) technologies have been actively researched and developed. The RF-MEMS signal selection devices are electromechanical signal selection devices using mechanical vibrations of micro-vibrators. In an RF-MEMS signal selection device, since electric oscillation of a high frequency signal is transduced into mechanical vibration of a micro-vibrator and an output signal is extracted therefrom as electric oscillation again, there is an advantage that the size of a resonator does not depend on the electric length so that the signal selection device can be miniaturized. In addition, the RF-MEMS signal selection device can be manufactured in a process having good affinity to an RF-IC. It is therefore possible to build the signal selection device in the RF-IC. The RF-MEMS signal selection device is expected as a technique which will make a large contribution to miniaturization of a wireless unit.
For example, an electromechanical signal selection device using a GHz-band micro-vibrator is disclosed in Non-Patent Document 1. According to this Non-Patent Document 1, a micro-vibrator like a disc is arranged on a silicon substrate so as to realize an electromechanical resonator with a center frequency of 1.14 GHz using a mechanical resonance phenomenon of the micro-vibrator. The mechanism of signal selection will be described. By virtue of a high frequency signal input from a signal input port to a driving electrode, an electrostatic force is applied between the driving electrode and the micro-vibrator so as to excite the micro-vibrator with the frequency of the high frequency signal. When a signal with a frequency equal to the mechanical self-resonant frequency of the micro-vibrator is input, the micro-vibrator is excited so greatly that the electrostatic capacity changes in accordance with a change of the distance between the micro-vibrator and a sensing electrode. Then, due to a voltage applied to the micro-vibrator, the mechanical vibration of the micro-vibrator is extracted as electric oscillation by the sensing electrode, and output from the sensing electrode to a signal output port. That is, only a signal with a frequency set by the self-resonant frequency of the micro-vibrator can be output selectively.
Presently, there is an attempt to make the applicable frequency higher and make the Q value (Quality Factor) higher in electromechanical signal selection devices. In order to attain a high frequency in the applicable frequency, it is necessary to make the self-resonant frequency of a micro-vibrator higher. To this end, a method of reducing the size of the micro-vibrator or a method of using a harmonic mode of the micro-vibrator can be considered.
As the micro-vibrator becomes finer from a micrometer order to a nanometer order, vibration thereof becomes extremely slight and close to a noise level of quantum vibration or thermal vibration. It is therefore necessary to obtain a supersensitive vibration sensing method by which vibration close to a quantum limit can be sensed.
Non-Patent Document 1: J. Wang, et al., IEEE RFIC Symp., 8-10 June, pp. 325-338, 2003.